Weather Watch

It’s hasn’t been too hot this summer in SLO. Here’s why that’s about to change — and when

If you were asked last year to predict the summer weather of 2019 by merely utilizing the historical data from Paso Robles, San Luis Obispo and the Santa Maria airports, your forecast would be nearly perfect.

Typically, the normal high temperature for Paso Robles in June and July is 90 degrees. As of Friday, the average high temperature at the Paso Robles Airport was 90 degrees. San Luis Obispo usually sees a high of 76 degrees, and if you guessed 76 degrees up to this date, you would be correct. Santa Maria’s Airport average high in June and July is about 72 degrees, but so far, it’s been slightly warmer at 74 degrees.

Up to now, it’s been the quintessential Central Coast summer weather pattern, but a change may be on way. Here’s why:

Earlier this summer, the jet stream buckled over the West Coast, producing a series of upper-level, low-pressure troughs along the coastline. The troughs decreased the amount of subsidence, or sinking of the air mass, that generally occurs during the summer. That, in turn, allowed a deeper marine layer to develop, producing milder temperatures.

Also, slightly stronger northwesterly winds created a more considerable amount of upwelling of deep and cold seawater to the ocean’s surface along the beaches, cooling the air above it.

However, a couple of weak monsoon events this July diminished the marine layer and the northwesterly winds counteracting the cooling, producing warmer air temperatures. This was important in keeping average air temperatures at seasonal levels. You see, over the years, I have seen air temperature at the Diablo Canyon Power Plant never reach the 60-degree mark on summer days, remaining in the 50s under mostly overcast skies.

Over the past few weeks, the long-range models have been indicating that a more stubborn and stronger ridge of high pressure will build over the West Coast, which could produce above seasonal temperatures through September.

Looking further out, the Climate Prediction Center is advertising that warmer than typical conditions will continue through October.

Even though the days are getting shorter as we move toward autumn, which should produce cooler temperatures, our atmosphere is a lot like a massive freight train with tons of momentum; it takes a lot of energy to get it going in spring and takes months to cool it off in fall. In fact, the warmest months for many Central Coast locations historically occur in August and September.

Often in fall, an area of high pressure builds at the surface over the Great Basin — the space between the Sierra Nevada range to the west and the Rocky Mountains to the east — and begins to dominate our coastal weather.

This condition usually produces Santa Lucia (northeasterly/offshore) winds, especially during the night and morning hours. Like a semi-truck rolling down a mountain grade, air from the higher elevations flow downward along the mountain slopes toward the Pacific Ocean, pulled by the never-ending force of gravity. These downslope winds are technically called katabatic winds, from the Greek word katabatikos, which means “going downhill.”

As the air mass descends the side of the mountain range, it warms at the rate of about 5.5 degrees Fahrenheit per 1,000 feet of descent. Meteorologists refer to this rate of warming as the dry adiabatic lapse rate. If the air is warm at the top of the mountain range, it can be sizzling hot and bone dry by the time it reaches the valleys below.

These winds can also create bone-dry relative humidity levels and counterintuitive time of peak temperatures, for example, in the early morning. These winds from the land out to the Pacific will push the low coastal clouds out to sea, leaving behind fog free skies.

If the long-range models verify, this upcoming August and September could be hot indeed. The forecast lower relative humidity levels and warmer than normal temperatures combined with large amounts of vegetation risk from last year’s abundant rainfall will increase the risk of wildfires. To prepare for wildfires, please visit www.pge.com.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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